Does urbanization explain differences in interactions between an insect herbivore and its natural enemies and mutualists?

Local and landscape factors shape alpha and beta trophic interaction diversity in urban gardens

Promoting urban green spaces is an effective strategy to increase biodiversity in cities. However, our understanding of how local and landscape factors influence trophic interactions in these urban contexts remains limited. Here, we sampled cavity-nesting bees and wasps and their natural enemies within 85 urban gardens in Zurich (Switzerland) to identify factors associated with the diversity and dissimilarity of antagonistic interactions in these communities. The proportions of built-up area and urban green area at small landscape scales (50 m radius), as well as the management intensity, sun exposure, plant richness and proportion of agricultural land at the landscape scale (250 m radius), were key drivers of interaction diversity. This increased interaction diversity resulted not only from the higher richness of host and natural enemy species, but also from species participating in more interactions. Furthermore, dissimilarity in community structure and interactions across gardens (beta-diversity) were primarily influenced by differences in built-up areas and urban green areas at the landscape scale, as well as by management intensity. Our study offers crucial insights for urban planning and conservation strategies, supporting sustainability goals by helping to understand the factors that shape insect communities and their trophic interactions in urban gardens.

The changing dynamics of ant-tree cholla mutualisms along a desert urbanization gradient

Urbanization, among the most widespread and multifaceted anthropogenic change drivers, exerts strong influences on a diversity of ecological communities worldwide. We have begun to understand how urbanization affects species diversity, yet we still have limited knowledge about the ways that species interactions are altered by urbanization. We have an especially poor understanding of how urbanization influences stress-buffering mutualisms, despite the high levels of multivariate stress that urban organisms must overcome and the importance of these interactions to the fitness of many organisms. In this study, we investigated the effects of urbanization on a mutualism between tree cholla cacti (Cylindropuntia imbricata) and visiting ants. We first examined how plant size, ant species composition, and ant activity varied on C. imbricata across an urbanization gradient (urban, suburban, wild) in and around Albuquerque, NM. Ant species composition and activity varied significantly across the urbanization gradient, with ant communities from wildlands having the highest activity and the most dissimilar species composition compared to both suburban and urban sites. In contrast, plant size remained constant regardless of site type. We then experimentally assessed how nectar levels influenced ant aggressive encounters with proxy prey (Drosophila melanogaster larvae) on C. imbricata across urban and wild sites. Ants were more likely to discover, attack, and remove proxy prey in wild sites compared to urban sites; they also performed these behaviors more quickly in wild sites. Nectar supplementation had weaker effects on ant aggression than urbanization, but consistently increased the speed at which aggressive behaviors occurred. Future studies that examine nectar quality and herbivorous arthropod abundance may help explain why this strong difference in ant composition and aggression was not associated with lower plant fitness proxies (i.e. size traits). Nevertheless, this study provides unique insight into the growing body of work demonstrating that mutualisms vary significantly across urbanization gradients.

Fine-scale monitoring and mapping of biodiversity and ecosystem services reveals multiple synergies and few tradeoffs in urban green space management

Urban watersheds can play a critical role in supporting biodiversity and ecosystem services in a rapidly changing world. However, managing for multiple environmental and social objectives in urban landscapes is challenging, especially if the optimization of one ecosystem service conflicts with another. Urban ecology research has frequently been limited to a few indicators - typically either biodiversity or ecosystem service indices - making tradeoffs and synergies difficult to assess. Through a recently established watershed-scale monitoring network in Central Texas, we address this gap by evaluating biodiversity (flora and fauna), habitat quality, and ecosystem service indices of urban green spaces across the watershed. Our results reveal substantial heterogeneity in biodiversity and ecosystem service levels and multiple synergies (stacked benefits or "win-wins"). For example, we found that carbon sequestration positively correlated with tree species richness and the proportion of native trees in a green space, indicating that biodiversity goals for increased tree diversity can also provide carbon sequestration benefits. We also documented correlations between green spaces with greater riparian forest cover and lower particulate matter (PM_2.5) concentrations and cooler temperatures. In addition, we found that bee and wasp species richness was positively correlated with carbon sequestration and human visitation rates, meaning that urban green spaces can optimize carbon sequestration goals without losing pollinator habitat or access opportunities for city residents. Overall, our results indicate that many aspects of habitat quality, biodiversity, and ecosystem services can be simultaneously supported in urban green spaces. We conclude that urban design and management can optimize nature-based solutions and strategies to have distinct positive impacts on both people and nature.

Urbanization hampers biological control of insect pests: A global meta-analysis

Biological control is a major ecosystem service provided by pest natural enemies, even in densely populated areas where the use of pesticides poses severe risks to human and environmental health. However, the impact of urbanization on this service and the abundance patterns of relevant functional groups of arthropods (herbivores, predators, and parasitoids) remain contested. Here, we synthesize current evidence through three hierarchical meta-analyses and show that advancing urbanization leads to outbreaks of sap-feeding insects, declining numbers of predators with low dispersal abilities, and weakened overall biological pest control delivered by arthropods. Our results suggest that sedentary predators may have the potential to effectively regulate sap-feeders, that are one of the most important pests in urban environments. A well-connected network of structurally diverse and rich green spaces with less intensive management practices is needed to promote natural plant protection in urban landscapes and sustainable cities.

The effects of urbanisation on ecological interactions

Cities are expanding worldwide and urbanisation is considered a global threat to biodiversity. Urban ecology has provided important insights on how urban environmental changes might affect individuals, populations, and species; however, we know little about how the ecological impacts of urbanisation alter species interactions. Species interactions are the backbone of ecological communities and play a crucial role in population and community dynamics and in the generation, maintenance and structure of biodiversity. Here, I review urban ecological studies to identify key mechanistic pathways through which urban environmental processes could alter antagonistic and mutualistic interactions among species. More specifically, I focus on insect predation, parasitoidism and herbivory, competition, insect host-pathogen interactions, and pollination. I furthermore identify important knowledge gaps that require additional research attention and I suggest future research directions that may help to shed light on the mechanisms that affect species interactions and structure insect communities and will thus aid conservation management in cities.

Phenotypic and genotypic divergence of plant‐herbivore interactions along an urbanization gradient

Urban environments provide challenging conditions for species survival, including increased temperatures, drought and pollution. Species can deal with these conditions through evolution across generations or the immediate expression of phenotypic plasticity. The resulting phenotypic changes are key to the performance of species and their interactions with other species in the community. We here document patterns of herbivory in Arabidopsis thaliana along a rural–urban gradient, and tested the genetic background and ecological consequences of traits related to herbivore resistance. Aphid densities increased with urbanization levels along the gradient while plant size did not change. Offspring of urban mothers, raised under common garden conditions, were larger and had a decreased trichome density and seed set but a higher caterpillar (Pieris brassicae) tolerance. In contrast, no urban evolution was detected for defences against aphids (Myzus persicae). Aphids reduced seed set more strongly in urban offspring, but this effect disappeared in second‐generation plants. In general, urban adaptations as expressed in size and caterpillar tolerance were found, but these adaptations were associated with smaller inflorescences. The maternal effect on the response of seed set to aphid feeding demonstrates the relevance of intergenerational plasticity as a direct ecological consequence of herbivory. Our study demonstrates that the urban environment interacts with the plant's genotype and the extended phenotype as determined by ecological interactions.

Downtown diet: a global meta-analysis of increased urbanization on the diets of vertebrate predators

Predation is a fundamental ecological process that shapes communities and drives evolutionary dynamics. As the world rapidly urbanizes, it is critical to understand how human perturbations alter predation and meat consumption across taxa. We conducted a meta-analysis to quantify the effects of urban environments on three components of trophic ecology in predators: dietary species richness, dietary evenness and stable isotopic ratios (IRs) (δ¹³C and δ¹⁵N IR). We evaluated whether the intensity of anthropogenic pressure, using the human footprint index (HFI), explained variation in effect sizes of dietary attributes using a meta-regression. We calculated Hedges' g effect sizes from 44 studies including 11 986 samples across 40 predatory species in 39 cities globally. The direction and magnitude of effect sizes varied among predator taxa with reptilian diets exhibiting the most sensitivity to urbanization. Effect sizes revealed that predators in cities had comparable diet richness, evenness and nitrogen ratios, though carbon IRs were more enriched in cities. We found that neither the 1993 nor 2009 HFI editions explained effect size variation. Our study provides, to our knowledge, the first assessment of how urbanization has perturbed predator-prey interactions for multiple taxa at a global scale. We conclude that the functional role of predators is conserved in cities and urbanization does not inherently relax predation, despite diets broadening to include anthropogenic food sources such as sugar, wheat and corn.

Land sharing between cultivated and wild plants: urban gardens as hotspots for plant diversity in cities

Plant communities in urban gardens consist of cultivated species, including ornamentals and food crops, and wild growing species. Yet it remains unclear what significance urban gardens have for the plant diversity in cities and how the diversity of cultivated and wild plants depends on the level of urbanization. We sampled plants growing within 18 community gardens in Berlin, Germany to investigate the species diversity of cultivated and wild plants. We tested species diversity in relation to local and landscape-scale imperviousness as a measure of urbanity, and we investigated the relationship between cultivated and wild plant species within the gardens. We found that numbers of wild and cultivated plant species in gardens are high – especially of wild plant species – independent of landscape-scale imperviousness. This suggests that all community gardens, regardless of their urban contexts, can be important habitats for plant diversity along with their role in urban food provision. However, the number of all species was negatively predicted by local garden scale imperviousness, suggesting an opportunity to reduce imperviousness and create more habitats for plants at the garden scale. Finally, we found a positive relationship between the number of cultivated and wild growing species, which emphasizes that community gardens present a unique urban ecosystem where land sharing between cultivated and wild flora can transpire. As the urban agriculture movement is flourishing worldwide with gardens continuously and spontaneously arising and dissipating due to urban densification, such botanical investigations can support the argument that gardens are places for the reconciliation of plant conservation and food production.

Urbanization and plant pathogen infection interact to affect the outcome of ecological interactions in an experimental multitrophic system

Urbanization can change interactions in insect communities, and the few studies of tritrophic interactions in urban settings focus on interactions between plants, herbivorous insects and their mutualists and natural enemies. Plant pathogen infection is also widespread and common, and infection may also alter such interactions, but we have no understanding of whether the ecological consequences of pathogen infection vary with urbanization. Using replicated aphid colonies on experimental plants, we investigated how infection by the plant pathogen Botrytis cinerea influences interactions between plants, aphids and the aphid natural enemies and ant mutualists in highly urbanized, suburban and rural study sites. Aphid and natural enemy abundance were highest in the suburban site, while mutualist ants were most abundant in the urban site, reversing the usual positive density-dependent relationship between natural enemies and aphids. The effect of pathogen infection varied with trait and site, mediated by natural enemy preference for hosts or prey on uninfected plants. The effect of infection on aphid abundance was only seen in the suburban site, where natural enemies were most abundant on uninfected plants and aphid numbers were greatest on infected plants. In the urban site, there was no effect of infection, while in the rural site, aphid numbers were lower on infected plants. Uninfected plants were smaller than infected plants and differed between locations. This study suggests that the effects of urbanization on ecological interactions may become more complex and difficult to predict as we study ecological assemblages and communities at greater levels of structural complexity.

Trait-Environment Relationships Reveal the Success of Alien Plants Invasiveness in an Urbanized Landscape

Urban areas are being affected by rapidly increasing human-made pressures that can strongly homogenize biodiversity, reduce habitat heterogeneity, and facilitate the invasion of alien species. One of the key concerns in invaded urban areas is comparing the trait–environment relationships between alien and native species, to determine the underlying causes of invasiveness. In the current study, we used a trait–environment dataset of 130 native plants and 33 alien plants, recorded in 100 plots covering 50 urban areas and 50 non-urban ones in an urbanization gradient in the arid mountainous Saint-Katherine protected area in Egypt. We measured eleven morphological plant traits for each plant species and ten environmental variables in each plot, including soil resources and human-made pressures, to construct trait–environment associations using a fourth-corner analysis. In addition, we measured the mean functional and phylogenetic distances between the two species groups along an urbanization gradient. Our results revealed strongly significant relationships of alien species traits with human-made pressures and soil resources in urban areas. However, in non-urban areas, alien species traits showed weak and non-significant associations with the environment. Simultaneously, native plants showed consistency in their trait–environment relationships in urban and non-urban areas. In line with these results, the functional and phylogenetic distances declined between the aliens and natives in urban areas, indicating biotic homogenization with increasing urbanization, and increased in non-urban areas, indicating greater divergence between the two species groups. Thereby, this study provided evidence that urbanization can reveal the plasticity of alien species and can also be the leading cause of homogenization in an arid urban area. Future urban studies should investigate the potential causes of taxonomic, genetic, and functional homogenization in species composition in formerly more diverse urbanized areas.

Urbanization alters the abundance and composition of predator communities and leads to aphid outbreaks on urban trees

Urbanization can affect arthropod abundance in different ways. While species with narrow habitat range and low dispersal ability often respond negatively to urban environments, many habitat generalist species with good dispersal ability reach high densities in city centers. This filtering effect of urban habitats can strongly influence predator-prey-mutualist interactions and may therefore affect the abundance of predatory and phytophagous species both directly and indirectly. Here, we assessed the effect of urbanization on aphids, predatory arthropods, and ants on field maple (Acer campestre) trees in and around the city of Budapest, Hungary. We used the percentage of impervious surfaces within a 500 m radius of each site as an index of the degree of urbanization. We found that the abundance of aphids increased with increasing level of urbanization. However, abundance of predatory arthropods and occurrence of poorly dispersing species within the predator community were negatively related to urbanization, and we identified these two independent factors as significant predictors of aphid abundances. The abundance of ants decreased with urbanization, and contrary to our expectations, did not affect the abundance pattern of aphids. Our results suggest that urbanization, by altering the abundance and composition of predator communities, can disrupt biological control of aphid populations, and thus may contribute to the aphid outbreaks on urban trees.

Map-A-Mole: Greenspace Area Influences the Presence and Abundance of the European Mole Talpa europaea in Urban Habitats

Simple Summary

The European mole is a burrowing mammal which is widely distributed across Britain and much of continental Europe. Its presence is readily confirmed by the presence of molehills, which contain the spoil heaps left behind as the mole digs its underground tunnels. Despite being easy to record, there are very few studies of moles in an urban environment. We asked how area of greenspace (largely parks, recreation areas, nature reserves and playing fields), distance to the nearest patch, human disturbance, how long the green patch had been isolated for, and degree of urban construction around the patch affected mole presence and abundance. We found that patch size affected mole presence, with a minimum greenspace of approximately 10 ha required. Where moles were found, larger patches had more signs of moles and surprisingly, mole abundance was also associated with the degree of urban construction around the greenspace. This result shows how urban planning can affect the presence of unusual species, such as the European mole.

Abstract

The European mole Talpa europaea is common across much of Britain. It has a unique fossorial lifestyle, and evidence of its presence is readily identified through the presence of characteristic molehills. Although molehills are often a common sight in urban greenspaces, moles are remarkably understudied, with very few studies to date exploring the urban ecology of moles. Here, we investigate if factors such as greenspace (largely urban parks and playing fields) area, intensity of management, distance to nearest patch, amount of time the patch had been isolated from other green patches, and the amount of urbanization (constructed surfaces) surrounding the patch, influence the distribution and abundance of urban moles. Mole signs (hills and surface runs) were counted in all discrete urban greenspaces (excluding domestic gardens and one private golf course) within an 89.5 km² area in the UK town of Reading. We found that 17 out of 59 surveyed sites contained moles, with their presence being recorded in greenspaces with a minimum patch area of approximately 0.1 km² (10 ha). Where present, the abundance of mole territories in the greenspaces was associated with both the area of greenspace and degree of urbanization within 150 m of the patch boundary. While the former was not surprising, the latter outcome may be a consequence of sites with an increased risk of flooding being home to fewer moles, and the surrounding area is also less likely to be built upon. This case study highlights how choices made in designing urban green infrastructure will determine which species survive in urban areas long into the future.

Effects of urbanisation and landscape heterogeneity mediated by feeding guild and body size in a community of coprophilous beetles

Although the impacts of urbanisation on biodiversity are well studied, the precise response of some invertebrate groups remains poorly known. Dung-associated beetles are little studied in an urban context, especially in temperate regions. We considered how landscape heterogeneity, assessed at three spatial scales (250, 500 and 1000 m radius), mediates the community composition of coprophilous beetles on a broad urban gradient. Beetles were sampled using simple dung-baited traps, placed at 48 sites stratified across three distance bands around a large urban centre in England. The most urban sites hosted the lowest abundance of saprophagous beetles, with a lower mean body length relative to the least urban sites. Predicted overall species richness and the richness of saprophagous species were also lowest at the most urban sites. Ordination analyses followed by variation partitioning revealed that landscape heterogeneity across the urban gradient explained a small but significant proportion of community composition. Heterogeneity data for a 500-m radius around each site provided the best fit with beetle community data. Larger saprophagous species were associated with lower amounts of manmade surface and improved grassland. Some individual species, particularly predators, appeared to be positively associated with urban or urban fringe sites. This study is probably the first to examine the response of the whole coprophilous beetle community to urbanisation. Our results suggest that the response of this community to urbanisation matches expectations based on other taxonomic groups, whilst emphasising the complex nature of this response, with some smaller-bodied species potentially benefitting from urbanisation.

Urbanisation alters ecological interactions: Ant mutualists increase and specialist insect predators decrease on an urban gradient

The modification of habitats in urban areas is thought to alter patterns of species interactions, by filtering specialist species and those at higher trophic levels. However, empirical studies addressing these hypotheses remain limited in scope and number. This work investigates (1) how main urban land uses affect predator-prey and mutualistic interactions, and (2) how specialist and generalist predators respond to size and availability of urban green spaces. In a large town in the UK, experimental colonies of ant-attended Black bean aphid Aphis fabae and non-ant-attended Pea aphid Acyrthosiphon pisum were monitored over two years. Ants were more frequently found in highly urbanised sites; however mutualistic ants were also more often encountered when the habitat was more plant diverse. Aphids were not affected by urban land uses, but A. fabae numbers were positively related to the presence of mutualists, and so indirectly affected by urbanisation. Predators were the only group negatively affected by increased urbanisation, and specialist species were positively related to increased proportion of urban green areas within the habitats. While this work supports the hypothesis that specialist predators are negatively affected by urbanisation, we also show that a fundamental ecological interaction, mutualism, is affected by urbanisation.

Urban tree pests and natural enemies respond to habitat at different spatial scales

Trees provide many ecosystem services in our urban environments. However, city trees are often stressed by pests that are typically higher than those in nearby natural areas. Our research highlights a potential mismatch in scale between the habitat elements that affect the densities of pests and their natural enemies on city trees. We tested a well-known ecological concept, the enemies hypothesis, in the city, where relationships of pests and their enemies have not been thoroughly studied. To test our hypothesis that natural enemies and aphid predation services on urban trees increase with more local structural complexity around trees, we collected data on crape myrtle trees on NC State University’s campus from 2016 to 2017. We measured local structural complexity of vegetation around study trees, quantified impervious cover among other urban habitat elements, collected crape myrtle aphids (Tinocallis kahawaluokalani) and their natural enemies on trees, and performed predation experiments. We found that aphid abundance was positively correlated with more impervious cover within 100 m of crape myrtle trees. Alternatively, greater local structural complexity within the 10 × 10 m area around crape myrtles correlated with a higher abundance of natural enemies. Aphid predation was mostly predicted by local structural complexity and impervious cover within 20 m of crape myrtle trees. Together, these findings suggest that although the impervious nature of our cities may mean higher densities of some pests, local landscapes around trees can play an important role in maintaining natural enemies and predation services that help regulate pest populations.

Urban plants and climate drive unique arthropod interactions with unpredictable consequences

Urban areas, a rapidly expanding land cover type, are composed of a mix of impervious surfaces, ornamental plants, and remnant habitat, which alters abiotic conditions and affects arthropod community assemblages and trophic interactions. Importantly, these effects often reduce arthropod diversity and may increase, reduce, or not change individual species or trophic interactions, which affects human and environmental health. Despite the pace of urbanization, drivers and consequences of change in urban arthropod communities remains poorly understood. Here, we review recent findings that shed light on the effects of urbanization on plants and abiotic conditions that drive arthropod community composition and trophic interactions, with discussion of how these effects conflict with human values and can be mitigated for future urbanization.